In many appliications, in the field of precision mechanics, particularly in the field of automation, it is necessary to actuate mechanical components at a low speed and with high-precision movements.
It is particularly necessary, for instance in the field of automatic manipulators, "robots ", to carry out low movements of the parts, controlling accurately the positions reached and insuring a high repeatability of the motions to ensure that it is possible to come back to a certain position by reversal of the motion previously executed.
In order to perform such motions or actuations, electric motors are commonly used, such motors are coupled with reduction units in order to obtain the required speed of motion.
Such reducing units have to meet a number of requirements, in order to accommodate such applications. Among these requirements are reduced space occepancy, limited internal friction, high reduction ratio, and absence of play. In order to achieve reduced friction, it is necessary to limit the number of gears used, while the high reduction ratio requires several reduction steps and therefore the use of many gears.
It is presently possible to satisfy such requirements by using an epicyclic speed reducer, but reducers of this kind are of a complex structure and in any case do not allow for the desired absence of play in the transmission.
In fact, in a transmission consisting of a couple of gears, it is generally the situation that the torque is transmitted with rotation in one direction from one gear to the other, and the driving teeth of one gear each have one flank in contact with the corresponding flank of teeth of the other gear, while the opposite flank of the first tooth will be spaced by a certain value from the corresponding flank of the teeth of the other gear. This distance is due to the limited precision of the gear cutting, of the center to center distance between gears, and like parameters which create play which generally cannot be completely eliminated, particularly in industrial production without adding unacceptable cost.
This distance between corresponding flanks of opposite teeth in engagement is very small, but its existence leads to the fact that, when the rotational motion is reversed, before the new flanks transmitting the torque are in contact, there is a short stretch of free rotation (backlash) prevents a very accurate correspondence between the driving and driven gears. In a reducing unit, having several pairs of gears in mutual engagement, this phenomenon is amplified, and, as already mentioned, can be eliminated only through high precision in the manufacture of the gears and of their support members.
In an automatic manipulator, where the control of the position of the moving parts can be achieved by computing the rotations of the respective drive motors, the presence of play in the reducers for which high speed-reducing ratios are required, is particularly serious, since it can endanger the precision and require various and more complex methods of position control.